Use this command to display the rulers on the top and left side of the active image frame, or hide them. The rulers allow you to determine the size and position of image elements in calibrated geometrical units. As you move the pointer in the image frame, marks on the rulers indicate its coordinates. If you pan or scroll the image, the coordinates on the rulers will change accordingly. If you zoom the image, the decimal scales on the rulers will readjust for the best readability.
If you double-click the crosshair in the upper-left corner of the frame, the vertical axis will reverse its direction. To perform measurements, you can move the origin of the coordinate system to any place within the frame. To do this, just drag the crosshair in the upper-left corner of the frame to wherever you want the origin of the coordinate system to be. To have the rulers return to their default position, double-click the crosshair in the upper-left corner of the frame. If you enlarge (Zoom In) or reduce (Zoom Out) the active image, the ruler scale will change accordingly. If you double-click any ruler, the Spatial Calibration dialog box will appear.